For many years the deployment of telecommunication systems, for different standards and many frequency bands, has been realized to a large extent by placing radio base stations (RBS) in cellular networks covering large areas. An important link in a traditional radio base station architecture is between the active parts of the system (that is the digital and analog components of the system) to the passive parts (such as the filters and antennas). This high-power analog radio-frequency (RF) link is critical in the sense that it sometimes requires long cables of high quality and large dimensions, in order keep the unavoidable signal quality losses and power losses to a minimum. Such links suffer from the disadvantage of having high costs.
There has been a recent change to integrate the power amplifier and other RF blocks more closely with the physical antenna in order to avoid this critical link, and has resulted in what is termed an integrated antenna unit (IAU). The introduction of an IAU implies a change from RF feeders into a high-speed digital interface between a digital processing unit (DPU) and the IAU.
In order to implement a base station today with two or more frequency bands, several complete transmitters are combined on the analogue side after a transmission filter.
Thus, when implementing transmitters for multiple frequency bands, two or more transmitters are implemented in the analogue domain, one transmitter for each frequency band.
FIG. 1 shows a known dual band transmitter having two digital transmitters 1101, 1102 for handling first and second frequency bands, the digital transmitters feeding first and second digital pre-distortion units (DPDs) 1111, 1112. The output of each DPD 1111, 1112 is converted into an analogue signal by a respective one of first and second digital to analogue converters 1121, 1122. A separate analogue up-converter 1131, 1132 is provided for converting each analogue baseband signal into a respective analogue RF signal. Each analogue up-converter 1131, 1132 is controlled by a respective tunable local oscillator 1141, 1142. The outputs from the first and second analogue up-converters 1131, 1132 are combined in an analogue combiner 115, before being fed to a power amplifier 116 (which feeds an antenna or antenna array, not shown). It will be appreciated that the transmitter shown in FIG. 1 is bulky, since it comprises multiple DPDs 1111, 1112, multiple DACs 1121, 1122 and multiple analogue up-converters 1131, 1132.
Furthermore, since tuneable local oscillators 1141, 1142 are required per transmitter chain, these have the disadvantage in that each requires tuning control, additional space and increased power consumption.
FIG. 1 also shows a feedback path for feeding a dual-band DPD processing unit 117. The feedback path comprises an analogue splitter 118, which feeds first and second analogue down-converters 1191, 1192, which in turn feed first and second transmitter observation receivers (TORs) 1201, 1202. The outputs from the first and second TORs 1201, 1202 are used for controlling the DPDs 1111, 1112. The transmitter shown in FIG. 1 is therefore bulky because it also requires multiple analogue down-converters 1191, 1192 and multiple TORs 1201, 1202. Further, the multiple analogue down-converters 1191, 1192 in the TOR chain also require separate tuneable local oscillators.
It can be seen from the above that the conventional technology has a disadvantage in that the multi band transmitters become bulky, particularly as more and more frequency bands are introduced, and have low energy efficiency and increased manufacturing cost due to the fact that several complete RF transmitters or several transmitter components in the analogue domain are used to implement the multi band transmitters.